An old and cost effective method for joining heat exchanger tubes to thin cooling fins is simple mechanical expansion. As shown in co-assigned U.S. Pat. No. 4,228,573, round, cylindrical tubes are run perpendicularly through round holes in thin cooling fins, all held temporarily in a fixture. An expansion rod with a bullet shaped, generally conical head is then pushed through each unexpanded tube. The largest cross section of the expansion rod is designed to expand the outer surface of the tube tightly into the edge of the fin, thereby securing it, within a short cycle time. The end result is a solid, sound structure, without brazing or welding. While the process is simple and effective, a round tube is not as thermally efficient as a generally elliptical or oval cross section tube, which has a greater ratio of surface area to volume.
Tubes with elliptical or oval cross sections are typically brazed to the cooling fin, which is effective, but more expensive and time consuming than mechanical expansion. However, there are issued patents that propose various methods for mechanically expanding elliptical tubes. U.S. Pat. No. 3,603,384 shows an extruded tube that has a stadium shaped cross section, with flat top and bottom walls and a central internal partition. Initially, the top and bottom walls are folded inwardly, about deep internal surface grooves that serve as hinge lines. Then, a pressurized medium is introduced into the tube to pop the kinked walls back flat and tightly into the edge of the fin hole. It is unlikely that internal pressure could ever create a sufficiently tight expansion of the tube walls into the edge of the fin hole, and certainly not as efficiently as simply pushing an expansion tool through the tube.
More practical proposals simply expand a tube of elliptical cross section into a matching fin hole with a rod that has an expansion head with a continually increasing elliptical cross section. This is the logical and obvious extension of expanding a round tube with an expansion tool of continually increasing circular cross section. An example may be seen in U.S. Pat. No. 4,560,317. The problem with such an approach is that the elliptical cross section tube is not nearly so resistant to either internal burst pressure or external crushing forces as is a round tube. Therefore, the heat exchanger shown in the U.S. Pat No. 4,560,317 patent is suitable as a low pressure radiator, not a high pressure condenser. Another patent discloses what might be referred to as a reverse ellipse, an oval tube with concave, rather than convex walls. It would also be expanded with a simple tool having a matching cross section. The reverse shape is claimed to be more pressure resistant, but as with any mechanically expanded, non-round tube, it cannot have integral internal strengthening walls, unless they are brazed in later. Another patent, U.S. Pat. No. 4,692,979 expands the tube with an elliptical expander, and then puts an unbonded spacer down the middle of the expanded tube, in the same location as the central tube partition of U.S. Pat. No. 3,603,384. While an unbonded spacer would resist crushing, it would not, of course, do anything at all to resist internal burst pressure. The design would, therefore, not be suitable for a high pressure condenser, either.
The lack of strength of mechanically expanded elliptical tubes highlights the real objective of the internal pressure tube expansion process disclosed in U.S. Pat. No. 3,603,317, which is to allow the use of the kind of internal, integral strengthening partition that is not possible with known methods of mechanical expansion. In summary, the current state of the art of non-brazed elliptical tubes offers a choice between inefficient tube expansion or tubes with insufficient strength.